1. Field of the Invention
The present invention relates in general to the field of signal processing, and more specifically to a system and method for processing signals with a multi-stage amplifier having multiple fixed and variable voltage rails.
2. Description of the Related Art
Many electronic devices utilize one or more amplifiers to amplify an electrical signal. For example, in an audio context, a microphone utilizes transducers to convert sound waves into a corresponding electrical signal. An audio and/or video playback device reads stored data and converts the data into an electrical signal. The electrical signal often has insufficient power to drive an output device such as an audio speaker. An amplifier amplifies the smaller electrical signal to a level sufficient to drive the output device. Conventional amplifiers utilize a single set of voltage rails to supply voltage rails to a multi-stage amplifier.
FIG. 1 depicts a closed loop amplifier circuit 100. Resistors R1 and R2 provide a voltage divider for input voltage VINM, and resistor R2 provides feedback resistance between the output and inverting terminals of operational amplifier 102. Resistors R3 and R4 provide a voltage divider circuit between input voltage VINP and reference voltage Vref. Operational-amplifier 102 drives the output voltage Vout so that the input voltages at the inverting and noninverting terminals of operational-amplifier 102 are approximately equal. A power supply supplies power to operational-amplifier 102 via voltage rails VDD and VSS to allow operational-amplifier 102 to operate.
A multi-stage amplifier includes multiple amplification stages. In at least one embodiment, operational-amplifier 102 includes multiple amplification stages. Each amplification stage utilizes power supplied by a power supply to amplify an input signal. The power supply provides a set of voltage rails, such as VDD and VSS, to each amplification stage of the multi-stage amplifier. In at least one embodiment, voltage rail VDD represents a higher voltage with respect to voltage rail VSS, and voltage rail VSS represents a negative voltage or ground.
FIG. 2, labeled prior art, depicts a signal processing system 200 with a conventional multistage amplifier 202. Signal source 208 provides analog input signal x(t), and signal source 208 can be any signal source such as a microphone or an audio and/or video device. Signal source 208 can also be any internal signal source within an integrated circuit. Amplifier 202 includes multiple, cascaded stages to successively amplify input signal x(t). Amplifier 202 generates analog output signal y(t). Amplifier stages 204 and 206 have respective gains g0 and g1. The overall gain of amplifier 202 is (g0+g1), and the overall gain relates the output signal y(t) to the analog input signal x(t), i.e. y(t)=(g0+g1)·x(t). Amplifier 202 supplies output signal y(t) to an output device, such as speaker 210.
Referring to FIGS. 2 and 3, a power supply 302 provides operating power to each of amplification stages 204 and 206 by providing voltage rails VDD and VSS to power supply nodes of amplification stages 204 and 206. Amplification stage 204 includes power supply nodes VDD IN and VSS IN to receive voltage supply rails VDD and VSS from power supply 302. Amplification stage 206 includes power supply nodes VDD OUT and VSS OUT to receive voltage supply rails VDD and VSS from power supply 302. As discussed with reference to FIG. 3, although each of amplification stages 204 and 206 has a unique set of power supply nodes to receive the set of voltage rails VDD and VSS, the multistage amplifier 202 is supplied by only one set of voltage rails, i.e. voltage rails VDD and VSS.
FIG. 3 depicts integrated circuit 304 connected to external power supply 302. Amplifier 202 is implemented, in this embodiment, as an integrated portion of integrated circuit 304. Power supply 302 is an external device that provides power to integrated circuit 304 through pads 306 and 308. Pad 306 receives the VDD voltage rail of power supply 302, and pad 308 receives the VSS voltage rail. Each of pads 306 and 308 has two conductive paths (310, 212) and (314, 316) connected to amplifier 202. Although the power supply nodes VDD IN and VDD OUT are distinctly labeled for purposes of identifying a specific path to amplifier 202, voltage rails VDD IN and VDD OUT are actually identical to each other. Likewise, although power supply nodes VSS IN and VSS OUT are distinctly labeled for purposes of identifying a specific path to amplifier 202, voltage rails VSS IN and VSS OUT are actually identical to each other.
Efficiency of an amplification stage, in terms of power loss, increases as a difference between an input signal voltage and voltage rail decreases. Thus, when the input signal voltage approximately equals the supplied voltage rail, the amplifier operates with a high degree of efficiency.
However, to amplify a signal, the voltage rails to amplification stage are set so that each amplification stage operates properly. In at least one embodiment, proper operation includes providing sufficient bias voltages to transistors within the amplification stage for operation in a predetermined mode, such as in a saturation mode, and providing sufficient input signal headroom. Input signal headroom represents a difference between an input signal level and a maximum input signal level that can be accommodated while still allowing the amplification stage to operate. Unless otherwise indicated, “input signal headroom” is referred to herein as “headroom”.
To provide sufficient headroom during operation, the voltage supply rails are fixed at specific voltage levels. During operation, input signals swing between minimum and maximum voltage levels. Thus, the efficiency of the amplifier decreases as the input signal decreases.